1. Field of the Invention
The present invention comprises an apparatus and method for lens casting and a gasket therefor. The present invention also encompasses a method for curing the casted lens and an apparatus and method for separating the cured lens from the molds used to form the lens.
2. Background Art
Glasses or spectacles must correspond to a person's prescription as well as to the person's morphological and psychological characteristics. The ophthalmic lenses for glasses are made of a transparent material, usually glass or plastic, and are of a size and shape to produce desired effects, namely, focusing the light for the person's eye to see clearly.
The lenses use a well-defined geometrical configuration which determines the their optical properties. The shape of each lens is characterized by three attributes: (1) the curvature of its two surfaces; (2) the thickness at its center and edges; and (3) its diameter. The two surfaces of a lens can use various geometric configurations, including the following shapes: spherical; cylindrical; toric; plano; aspheric (usually elliptical); and progressive. For example, the surface of a lens can have a constant radius along its different axes so that the surface is symmetrical, which is known as a spherical surface. The spherical lens surface mirrors the shape of a portion of a sphere in which all meridians have the same radius of curvature. The spherical surface may be either convex or concave.
Alternatively, the surface of the lens can have two axes, each having a different radius of curvature, so that the surface of the lens is asymmetrical. An astigmatic surface is an example of such an asymmetrical surface and is characterized by its two principal meridians having a different radius of curvature from each other. The meridian having the greatest radius of curvature is called the "axis," and the other meridian having the smaller radius is called the "perpendicular axis." Astigmatic lens surfaces predominantly include a cylindrical surface and a toric surface. A plano surface and aspheric surface are examples of other lens surfaces used in the art.
For the cylindrical surface, the principal meridians along the axis has an infinite radius of curvature, e.g., flat or straight, and the perpendicular axis has a radius of curvature which is the same as the circular radius of a cylinder. Thus, a concave cylindrical surface is shaped to complementarily receive a cylinder on the surface and a convex surface resembles the exterior surface of such a cylinder.
The toric surface resembles the lateral surface of a torus, e.g., shaped as the inner tube of a tire. Thus, a torus surface is similar to a cylindrical surface, but the longitudinal axis curves instead of being straight as for a cylindrical surface. The perpendicular axis or meridian on the toric surface has a radius of curvature smaller than the radius of the axis. As with a spherical and a cylindrical surface, a toric surfaces can be convex by having the shape of the exterior surface of a torus or, alternatively, may be concave by having the shape of the inner surface of a torus.
An astigmatic surface is used for a person with an ocular astigmatism, in which the cornea is elliptical instead of round. The orientation of the elongated portion of an astigmatic cornea varies from person to person. For example, one person may have an axis at five degrees, another at thirty degrees, and another at yet a different orientation. The axis of the surface of the lens must be oriented to align with the orientation of the elongated portion of the cornea.
Different lens surfaces can be used in combination. Often, the front surface of a lens is spherical and the back surface is spherical, cylindrical, or toric. The front surface can alternatively be a piano surface. The optimum combination of surfaces in a lens is determined by the optical properties, the proposed use, and the appearance of the lens.
In addition to shape, thickness is also an important characteristic of a lens. The glass or plastic used to form the lens is a factor in establishing the thickness. Many lens today are made from plastic because of its light weight, density, refractive index, and impact resistance. Examples of plastics used for lenses include methyl-methacrylate (a thermoplastic resin, which is better known by its trademark "Plexiglas".RTM. or "Perspex".RTM.) and dandiallyl glycol carbonate, which is also known as CR39.
CR39 is the most popular lens material used today, in part, because all lens types used in ophthalmic optics can be made from it. CR39 is a petroleum derivative of the polyester group, a family of polymerisable thermosetting resins. In production, a monomer is first obtained from CR39. The monomer, which is a limpid liquid with the viscosity of glycerine oil, remains in a liquid state in cold storage, but hardens after several months at room temperature. To form a lens, the liquid monomer is placed and contained in a volume defined by two molds and a gasket. Once the monomer is in the volume, the monomer is cured to form a hardened polymeric lens taking the shape of the molds.
The glass molds used to form polymeric lenses are important in CR39 lens manufacturing. Not only do the molds form the correct shape to the lens according to the optical characteristics required, but the surface qualities of the finished lens depends on the accuracy of the molds since the lens surfaces are a precise reproduction of the inner mold surfaces. Accordingly, the mold surfaces are prepared with extreme precision and, after manufacture, arc heat toughened to withstand the strain of the polymerization process.
An add power front mold, which forms a bifocal or trifocal portion to the lens, can also be used in forming lenses. The add power mold includes a segment curve, which is a concave depression cut into the concave half of the mold, to form the add power segment on the front surface of the lens. This segment curve produces a convex surface for the distance portion, together with a steeper convex surface for the reading add power segment.
The liquid monomer, as mentioned above, is placed into a volume defined by two molds and a gasket to form a lens. As shown in cross-section in FIG. 1, the prior art gaskets are known as T-gaskets G, which have a bore B and two ends that each complementarily receives a respective mold M. Different T-gaskets G are required to form varying power lenses because each T-gasket G sets a predetermined axial separation between molds M. That is, one T-gasket G sets the molds farther apart to form a lens of a greater power compared with another T-gasket G used to form a lower power lens. Accordingly, manufacturers must maintain separate T-gaskets for a +2 lens, another for a -3 lens, another for a -4 lens, etc.
One skilled in the art will also appreciate that forming an astigmatic surface in a T-gasket G requires that the ends of the gasket have the same shape as the inner surface of the mold M. For example, if the rear mold M forms a concave toric surface, then one end of the T-gasket G must have a complementary convex design to receive the mold M without leaking. And, different T-gaskets G must exist for each lens power using that mold shape.
Two manufacturing processes are used in making the lenses: direct polymerization and polymerization of a semi-finished lens. For the direct polymerization process, the top mold is removed and a nozzle directed into the mold cavity to fill the volume with monomer. The operator then positions the top mold to be aligned with the T-gasket so that excess monomer is squeezed out and air bubbles removed. The volume defined by the two polished molds and the gasket forms the shape of the lens when cured. The drawbacks of this prior art system include the handling, resulting mess, and wasted monomer. Also, some bubbles can still remain in the volume, which can ruin the formed lens. Additionally, this process is labor intensive and, accordingly, often performed in countries with an inexpensive labor pool.
To cure the monomer after the top mold is secured on the T-gasket, the filled gasket assemblies are stored in racks and put into an oven for fourteen to sixteen hours to undergo a controlled temperature cycle that provides the correct degree of polymerization. After this lengthy curing process is completed, the gasket assembly is removed and the lens is removed from between the molds. The lenses made using the direct polymerization process require a few finishing actions, such as edge trimming, annealing to eliminate casting stress, visual checks to eliminate lenses which might have defects, and lens power checking with a focimeter. Once the lens is finished, it is packed for shipping to a retail vendor or mounted in glasses for a consumer.
The second process, polymerization of a semi-finished lens, produces a lens known as a "semi." Unlike the direct polymerization lens, a semi lens has a concave, unfinished side that is surfaced after the curing process is completed. Thus, instead of forming the lens to be mounted into glasses with few finishing actions, the semi lens only has a single finished surface formed by a mold and the other surface is mechanically finished after the lens has been cured. The semi lenses, accordingly, are made in stages, in which one surface is finished by a mold and the other surface is machine finished after curing. The surface of the lens formed by a mold is usually the front spherical surface, with or without add power.
The unfinished side of a semi lens is usually surfaced using a special lathe or generator in the same way as glass lenses, but other abrasives are used. The polymeric lens is mounted on a circular holder and the surface generated by a diamond grinding wheel. The curvatures are controlled by the relative positions and angles of the diamond wheel in relation to the lens. This surface is then smoothed and finally polished with the appropriately-faced tool. Metal tools are used to smooth and polish, each surface configuration having its own tool. A great number of tools are, therefore, required to enable a complete range of lenses to be surfaced. Semi-finished lenses are generally put into stock and machined when needed, e.g., a customer with a specific prescription orders the lens.
Semi lenses are used instead of by direct polymerization lens for very powerful lenses, such as aphakic lenses or lenses with a very high cylindrical power. These powerful lenses cannot be made by direct polymerization because the difference in thickness between the center and the edges of the lens create large stresses that can break the glass molds with the T-gasket.
Another reason for making a semi lens is because of the numerous lenses required for different consumers, which is not conducive for mass production. For example, a prescription may require a certain add power on the front surface and an astigmatic back surface set at one of many different orientations. That is, the add power portion must be oriented so that the flat top is horizontal, but the orientation of the astigmatic surface varies as the elongated portion of the cornea differs from person to person. As one skilled in the art appreciates, numerous permutations exist for a specific add power and a given astigmatic back surface at different orientations. Mass production of an infinite variation of lenses is unfeasible and, accordingly, retail suppliers usually purchase a semi lens and machine the astigmatic surface immediately before sale.